An internal combustion engine comprises a plurality of cylinders, which are either arranged in line in a single row or are divided into two reciprocally angled rows. Generally, relatively low displacement engines (typically up to two liters) have a limited number of cylinders (usually four, but also three or five) arranged in line in a single row; conversely, higher displacement engines (more than two liters) have a higher number of cylinders (six, eight, ten or twelve) divided into two reciprocally angled rows (the angle between rows is generally from 60° to 180°).
A high displacement engine (more than two liters) is capable of generating a high maximum power, which however during normal driving on roads is rarely exploited; particularly when driving in cities, the engine generates a very limited power, which is a limited fraction of the maximum power in the case of a high displacement engine. It is inevitable that when a high displacement engine outputs limited power, such power output occurs at very low efficiency, and with a high emission of pollutants.
It has been proposed to deactivate some (usually half) of the cylinders in a high displacement engine when the engine is required to generate limited power; in this way, the cylinders which remain active may operate in more favorable conditions, increasing the total engine efficiency and reducing the emission of pollutants.
According to the currently proposed methods, in order to deactivate a cylinder, injection is cut off in the cylinder (i.e. the corresponding injector is not controlled) and either both the corresponding suction valves and the corresponding exhaust valves are maintained in an open position or only the corresponding suction valves are maintained in a closed position. A mechanical decoupling device is required to keep a valve in a closed position, the device being adapted to decouple the valve from the respective camshaft. However, such mechanical decoupling devices are complex and costly to make, particularly in high maximum revolution speed engines; furthermore, such mechanical decoupling devices inevitably entail increased weight of the moving parts, with consequent increase of inertial stress to which the distribution system is subjected.
Generally, in an engine whose cylinders are arranged in two rows, a respective throttle valve arranged upstream of an intake manifold of the row is associated with each row; furthermore, a respective catalyzer arranged downstream of an exhaust manifold of the row is associated with each row. It is convenient to deactivate all of the cylinders of a row in order to deactivate part of the engine cylinders; however, in this case the catalyzer associated with the deactivated row tends to cool down as it is no longer crossed by the hot exhaust gases from the row. When the row is reactivated, the catalyzer is cold and therefore presents very low efficiency for a significant, not negligible time.
U.S. Pat. No. 4,467,602A1 discloses a split engine control system operating a multiple cylinder internal combustion engine by using only some of the plurality of cylinders under light load conditions. The total number of cylinders are split into a first cylinder group which is always activated during engine operation and a second cylinder group which is deactivated under light load conditions. The engine is provided with an exhaust passage which consists of first and second upstream exhaust passages connected to the first and second cylinder group, respectively, and a common downstream exhaust passage; an exhaust gas sensor and a first catalytic converter are disposed in the first upstream exhaust passage, and a second catalytic converter is disposed in the common downstream exhaust passage.